Rubberized paint is commonly applied on plastic elements where it is desirable to have a tactile feel, such as on the keys or buttons of a keyboard or keypad, or to have a shock absorbing surface, such as on a lens cap or the contact surfaces of cameras, binoculars, and other equipment made of plastic. Applying a rubberized paint coating to a small surface area is relatively easy to manage. However, it is quite difficult to apply a rubber finish to a large surface area, particularly for equipment that is to be contantly handled, because the paint after drying tends to chip off under rough handling.
An example of rubberized paint that is currently sold commercially is Daiplacoat.TM. Soft BV type paint available from Dainichiseika Color and Chemical Manufacturing Co., Ltd., of Tokyo, Japan. For application of a film of about 20 microns thickness to a commonly used plastic such as ABS plastic, the specifications for the rubberized paint include tests for adhesion (Scotch tape test, 100 squares applied to 100 mm.sup.2), hardness under point pressure (JIS K-5400 6.14, grade B), resistance to heat (120 hrs. at 60.degree. C.) and humidity (72 hrs. at 95% R.H. and 40.degree. C.), thermal cycles (5 cycles, each for 4 hrs. at -20.degree. C. and 4 hrs. at 60.degree. C.), weathering (500 hrs. on Weather-O-Meter), and blocking (polyethylene sheet with 500 g loading for 24 hrs.). However, it is found that when the paint is applied to a plastic article having a relatively large surface area, such as a telephone handset unit and main body, the performance of the coating in terms of resistance to heat and humidity is only about half of the manufacturer's specification and chipping commonly occurred.
When chipping and poor surface quality occurs, the manufacturer suggests the use of a diluent or paint reducer to dilute the paint before spraying. A diluent recommended by the above-noted manufacturer is called "China Paint 50" reducer, and has a composition of 10% ethyl acetate, 35% methyl isobutyl ketone (MIBK), 25% xylene, 7.5% cellosolve acetate, 2.5% cyclohexanone, and 20% toluene. By spraying several successive coats of diluent-reduced paint, a more stable surface quality should be obtained. However, it is found that even with the use of the manufacturer's recommended diluent, the desired specifications for adhesion, hardness, and resistance to heat and humidity are not fully obtained.